System and method for high-speed laser detection of ultrasound

ABSTRACT

A system and method for laser light amplification provides amplification of a laser light beam emitted from a laser light source as low-amplification seed laser light signal. The low-amplification seed laser light signal is transmitted to an amplification component. The amplification component amplifies the low-amplification seed laser light signal by stimulating emissions of the population inversion provided by a pumping diode to generate an amplified laser light signal. The system and method further directs the amplified laser light signal to an output destination. The destination may be an object undergoing laser ultrasound testing. The amplified laser light may reflect with a modulation characteristic of a sound energy wave about the object. The reflected laser light may be collected by an interferometer and used in the detection and characterization of the sound energy wave. The result of the present invention is a system and method of operation providing higher pulse rates, improved pointing stability, and optionally variable pulse rates for a variety of uses, including for non-destructive laser ultrasonic testing of materials.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/344,298, filed Jun. 2, 1999 now U.S. Pat. No. 6,483,859,entitled: “System and Method for High-Speed Laser Detection ofUltrasound”, and is incorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to a system and method for laserlight amplification and, more particularly, to a system and method forproviding amplification of a laser beam emitted from a solid state laserthat provides higher pulse rates, improved pointing stability, andoptionally variable pulse rates for a variety of uses, including fornon-destructive laser ultrasonic testing of materials.

BACKGROUND OF THE INVENTION

Amplification of laser light is required for a variety of applications.Long haul telecommunication applications, such as those employing singlemode optical fiber, often require optical repeater/amplifiers to boostsagging signal levels. Material processing applications may require veryhigh power laser light to perform functions such as cutting of variousmaterials and preparation of material surfaces. Applications requiringintense energy pulses of laser light employ some configuration forproviding either time-varying optical amplification or intensitymodulation of laser light.

One method for amplifying a laser beam is to employ a laser medium whoseoptical gain may be controlled by optical pumping. Optical pumping of asolid state laser medium is a common and conventional method used tocreate a population inversion of energy states for laser applicationsrequiring high-gain. The laser medium providing high-gain, whenoptically pumped, may comprise a material such as neodymiumyttrium-aluminum garnet (Nd³⁺:YAG), neodymium glass (Nd³⁺:glass), Erbiumdoped optical fiber (Er³⁺:silica), or Ruby rods (Cr³⁺:Al₂O₃). Thesematerials are merely exemplary candidates for high-gain laser media, andthose skilled in the art will appreciate that any suitable materialcapable of maintaining an inverted population of energy states whenoptically-pumped may serve as an optical amplifier. Those laser mediautilizing Nd³⁺:YAG are common, given the substantial optical gain neardesired wavelengths near the 1.064:m range. Additionally, Nd³⁺:YAG lasermedia provide linearity of pumping rate with respect to invertedpopulation given its four-level transition system.

To saturate an entire laser medium with an inverted population throughoptical pumping, a conventional method is to distribute a large array oflaser diodes across the surface of the laser medium to form a pumpingarray. The light emitted from the individual laser diodes of the pumpingarray excites the laser medium and provide a very high optical gain forthe energy transition level of the optically-pumped, inverted populationwithin the high-gain laser medium, e.g., near the 1.064:m range forNd³⁺:YAG, near the 1.06:m range for Nd³⁺:glass, near the 0.6943:m rangefor Cr³⁺:Al₂O₃, near the 1.55:m range for Er³⁺:silica, etc.

An integrated approach to performing laser light amplification andgenerating optical pulses utilizes gain switching of a laser medium. Inthis method of providing a high energy pulsed laser beam, the opticalpumping of a high-gain laser medium itself is pulsed to generate a timevarying gain of the high-gain laser medium through which a laser beam ispropagating. This results in a pulsed output laser beam after anoriginal laser light source has traveled through the high-gain lasermedium that is being optically pumped in a time varying manner.

Each optical pumping cycle takes the high-gain laser medium through atransition which consists essentially of generating a sufficient energystate population through optical pumping to reach threshold foramplification. Before the optical pumping begins, the population ofenergy states is initially below threshold and optical amplificationdoes not occur. After the high-gain laser medium has operated in anamplification mode for some time, then optical pumping is switched off,and the energy state population is subsequently depleted. By turning offthe optical pumping, the population falls below threshold and theoptical amplification is interrupted until the optical pumping againresumes and the population of energy states again reaches threshold.Such a method provides for a pulsing of the conditions in which laserlight amplification may occur. Such a method is preferable to a methodwhich merely blocks a highly amplified laser beam in that designconsiderations need not include the potentially loss energy due to thedumping of electromagnetic energy into a shutter assembly. Many otheradvantages are inherent to the fact that the solution is electronic, notincorporating any mechanical components for a mechanical shutter system.

Another method for providing an electronic solution is to generate ahigh energy pulsed laser beam to maintain continuous optical pumping ofthe high-gain laser medium and to modulate the high-gain laser medium'sloss coefficient. One method to perform such loss switching is toelectronically modulate an optical absorber that is placed within theoptical resonator cavity next to the high-gain laser medium. Such aconfiguration will permit the user to control the loss of the laserlight traveling through the high-gain laser medium as opposed tocontrolling the rate at which optical pumping occurs. Those skilled inthe art will recognize a variety of methods for performing lossswitching of laser light contained within a the high-gain laser mediumincluding electrical modulation of an electro-optic crystal to performintensity modulation of the laser beam.

Such a method is an extension of the gain switching method as an opticalresonator's threshold energy state population difference is proportionalto the resonator's loss coefficient. In this method, the losscoefficient is modulated to provide intermittent periods when theoptical loss of the high-gain laser medium is prohibitively high tomaintain oscillation. This results is creating an increased energy statethreshold population to sustain oscillation, given the increased loss ofthe high-gain laser medium. Even though the energy state populationwould be sufficiently high for oscillation were the loss coefficient ofthe high-gain laser medium not increased, no optical amplification canoccur during the period when the loss coefficient is elevated.

When the loss is suddenly decreased during the transition of a pulsingcycle of the loss coefficient, the energy state population begins todeplete resulting from the decreased optical losses. The high-gain lasermedium will amplify the laser light during the period when the energystate population exceeds the threshold condition for oscillation duringthe period that the loss coefficient is minimum. However, as thepopulation continues to decrease, the population will eventually fallbelow the newly established energy state threshold for oscillationcorresponding to the period of time when the loss coefficient is at itsminimum during a modulation cycle.

These methods of performing electronic switching of either the gain orloss coefficients of the high-gain laser medium often employ flashlampoptical pumping. The use of such a light source for performing theoptical pumping presents some undesirable effects which significantlylimit performance of the high-gain laser medium in providing pulses oflaser light including the maximum pulse rate of the laser beam and theintensity with which the optical pumping must be performed. Suchinherent problems may present significant problems for applicationswhich require high pulse rates and suffer from limited power budgets.

Another problem that is introduced by the utilization of flashlamps toprovide optical pumping is the broad spectral width of flashlampproduced light may prove very inefficient in that a large proportion ofthe light produced by the flashlamp does not serve to generate theinverted population of energy states. Flashlamp light outside of thespectral density range required for generating the inverted populationis simply lost into the high-gain laser medium in the form of thermalheating. This heating of the high-gain laser medium may itself produceundesirable effects including beam pointing errors and self-focusing.The heating of the high-gain laser medium may increase to such levelsthat fracture of the solid-state crystals will limit the maximum peak oraverage power.

The pulse rate at which the laser amplifier may be switched is alsolimited by the physical properties of the flashlamps which provide theoptical pumping. The electrical switching of the flashlamps is oftenassociated with the thermal heating problems associated with theflashlamps themselves. This upper limit of pulse rate may also bedetermined in part by the intensity level at which the flashlamps mustoperate to generate an inverted energy state population above threshold.For example, if the energy transition of interest is near the peripheryof the spectral density of the flashlamp, the flashlamp may necessitateoperation at a very high power level to generate the invertedpopulation. Such a situation may at the very least limit the duty cycleof the pulse rate to avoid overheating of the flashlamps themselves.

Additionally, the flashlamps intrinsically possess a start up timeconstant before they begin optical pumping. They do not respondinstantaneously with the vertical transition of the electric signalwhich drives them. Consequently, the maximum pulse rate of the opticalamplifier may be limited by the time constant corresponding to the startup of the flashlamps. Another consequence of the intrinsic responselimitations of the flashlamps is a lower limit on the width of the pulsewhich may be generated using such a laser amplification system. Such aproblem stems from the similar characteristic of the flashlamps in thatthey are limited in the speed with which they may switch on and off. Theminimum pulse width which may be generated is often dictated by theminimum time in which the flashlamps may turn on and then turn off,including considering of the start up time constant of the flashlampsand evanescent decay of radiation from the flashlamps when turned off.

The present invention overcomes or eliminates the problems andlimitations of known systems and methods for detecting high-speedlaser-induced ultrasound to provide a system and method for laser beamamplification from a solid state laser that yields high pulse rates,improved pointing stability, and optionally variable pulse rates fornon-destructive laser ultrasonic testing of materials, as well as avariety of other uses.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided amethod for generating an amplified laser beam at a high pulse rate thatincludes generating a low-amplification seed laser light signal. Themethod further includes transmitting the low-amplification seed laserlight signal to an amplification component. The low-amplification seedlaser light signal is amplified in the amplification component bystimulating emissions of the population inversion that a pumping diodeprovides. The result of this amplifying step is to yield an amplifiedlaser light signal. The amplified laser light signal is then directed toan output destination. The light signal may scatter from the outputdestination and be collected in an interferometer.

The present invention provides a system and method for providingamplification of laser light from a solid state laser while maintainingthe physical properties of the laser light by minimizing amplificationinduced distortion. A seed laser possessing desired physical propertiesincluding a single longitudinal mode with a desired linewidth is passedthrough a high-gain laser medium. The high-gain laser medium may takethe form of a diode pumped rod or slab, among others. The high-gainlaser medium is optically pumped using a pumping array of laser diodesdistributed across the high-gain laser medium. The electric currentwhich drives the pumping array may be a time-varying signal whichconsequently provides time-varying optical gain and lasing conditionswithin the laser medium. The amplified laser beam may then be pulsed ata pulse rate corresponding to the frequency of the time-varying signalcomprising the electric current which drives the pumping array of laserdiodes.

The present invention may be employed in applications which require aparticularly narrow or pure spectral density such as applicationsinvolving optical interferometry which often require a singlelongitudinal mode having a very stable center frequency and linewidth.For such applications in which the purity of the initial seed laser isamplified to generate a pulse stream of laser light having a desiredintensity level, duty cycle and pulse rate, an optical isolationassembly may be used to minimize back reflection of the laser light intothe seed laser light source. Undesirable parasitic feedback may corruptthe seed laser source resulting in deleterious performance of the seedlaser including amplitude noise and multimode operation. Such effectsmay be disastrous for those applications requiring a spectrally purelaser source. One method, for providing optical isolation of a laserbeam which minimizes back reflections into the original light sourcecomprises a Faraday rotator and two polarizers which provide for lightpropagation in only one direction through the optical isolation assemblyby using the non-reciprocal rotation of a polarized lightwave providedby a Faraday rotator.

Additional optical isolation assemblies may be included in the inventionfor providing beam splitting of the original laser beam for directingthe amplified, pulsed output laser beam or directing of the originallaser beam through the high-gain laser medium multiple times for evengreater amplification than a single pass through the high-gain lasermedium.

The present invention generates an amplified and pulsed laser beam whichpossesses similar physical properties of the original seed laserincluding a desired center frequency and linewidth for use within anoptical interferometer to perform ultrasonic detection. Additionally,the output intensity of the pulsed, amplified laser beam may bemodulated to extend the dynamic range of detection within an opticalinterferometric system. Electro-optic modulators comprising Pockels orKerr effect crystals provide intensity modulation of a laser beam.

The present invention may perform optical pumping of a high-gain lasermedium using a pumping array of laser diodes. This method permitsoptical pumping within a very narrow wavelength regime selected by theuse of appropriate laser diodes to minimize optically-induced thermalheating of the laser medium. This advantage is provided primarily by thefact that the appropriate choice of laser diodes which comprise thepumping array may be chosen to optically pump within a specificwavelength regime thereby not incurring significant thermal heating byradiation bombardment of the high-gain laser medium with optical pumpingoutside of the energy transition level of interest.

The present invention may provide for maximizing the pulse rate andcontrolling the pulse width of the resulting amplified pulsed laser beamby performing the optical pumping using a pumping array comprising amultiplicity of laser diodes. The maximum pulse rate for the presentinvention will be determined largely by the speed at which the laserdiodes may be turned on and turned off and the allowed duty cycle.

The present invention may minimize amplification induced distortion of aseed laser beam. Many applications, including optical interferometry,require a highly amplified beam with a uniform wavefront and goodpointing stability.

The present invention may provide for intensity modulation of the outputlaser beam thereby expanding the detection dynamic range withininterferometric systems.

As such, a system for generating a laser for use with an interferometeris described. Other aspects, advantages and novel features of thepresent invention will become apparent from the detailed description ofthe invention when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numerals indicate like features and wherein:

FIG. 1A shows a polarization selective assembly comprising an opticalisolator;

FIG. 1B shows a polarization selective assembly comprising a four portoptical device providing polarization selective directing of a laserbeam;

FIG. 2 shows a configuration of a laser light source capable ofpreventing optical feedback and varying the intensity variation of thelaser light by rotating a half waveplate;

FIG. 3 shows another configuration of a laser light source capable ofpreventing optical feedback and varying the intensity variation of thelaser light using an electro-optic modulator;

FIG. 4 shows one possible embodiment of the invention comprising a fourpass, dual rod laser media pulsed laser light source;

FIG. 5 shows another possible embodiment of the invention comprising adual pass, dual rod laser media pulsed laser light source;

FIG. 6 shows another possible embodiment of the invention comprising afour pass, single slab laser medium pulsed laser light source;

FIG. 7 shows another possible embodiment of the invention comprising afour pass, dual slab laser media pulsed laser light source;

FIG. 8 shows another possible embodiment of the invention comprising aneight pass, single slab laser medium pulsed laser light source;

FIG. 9A shows one alternative embodiment of the present invention; and

FIG. 9B shows still a further alternative embodiment of the presentinvention.

FIG. 10 shows a schematic block diagram of an exemplary embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are illustrated in the FIGUREs,like numerals being used to refer to like and corresponding parts of thevarious drawings.

The present invention provides a system and method for providingamplification of laser light from a solid state laser while maintainingthe physical properties of the laser light while minimizingamplification induced distortion. A seed laser possessing desiredphysical properties including a single longitudinal mode with a desiredlinewidth is passed through a high-gain laser medium. The centerfrequency of the seed laser source may be chosen appropriately as toperform within specific applications such as optical interferometrywhich require very coherent light. The high-gain laser medium may be,for example, optically pumped using a pumping array of laser diodesdistributed across the high-gain laser medium. The electric currentwhich drives the pumping array may be a time-varying signal whichconsequently provides time-varying optical gain and lasing conditionswithin the laser medium. The amplified laser beam may then be pulsed ata pulse rate corresponding to the frequency of the time-varying signalcomprising the electric current which drives the pumping array of laserdiodes.

FIG. 1A shows a polarization selective assembly comprising an opticalisolator 10. A typical arrangement of an optical isolator passes a laserbeam 18 through a first polarizer 12. The polarized light then passesthrough a Faraday rotator 14. A Faraday rotator provides opticalrotation of a polarized light way in a non-reciprocal fashion. That isto say, polarized light passes through the Faraday rotator will rotatein one and only one direction regardless of the direction of propagationof the laser beam through the material. For example, a laser beam 18traveling through Faraday rotator 14 rotates clockwise as it propagatesin the direction of the arrows of laser beam 18, then a laser beam 18traveling in the opposite direction of the arrows of laser beam 18 willalso rotate clockwise. Faraday rotators 14 are well known to thoseskilled in the art, and may comprise a number of materials includingyttrium-iron-garnet (YIG) or terbium-gallium-garnet (TGG).

The first polarizer 12 transmits only P-state light from the input 18.After traveling through Faraday rotator 14, laser beam 18 is rotated45°. Following the Faraday rotator is a half waveplate 16 which providesan additional 45° of polarization rotation in the opposite direction ofFaraday rotator 14. Light polarized in a P-state traveling in thedirection of the arrows has a net rotation of zero. A half waveplate andits operation are well known to those skilled in the art. Lightpolarized in a p-state traveling in the direction of the arrows of laserbeam 18 through the optical isolator 10 travels unencumbered, yet light(possibly unpolarized, such as from random scattering) traveling in theopposite direction of the arrows of laser beam 18 through the opticalisolator 10 is blocked. This stems from the fact that the Faradayrotator exhibits non-reciprocal angular rotation of a polarized lightbeam. Light traveling in the opposite direction of the arrows of laserbeam 18 is polarized by the a polarizer 12, then pass through a halfwaveplate 16 and then through the Faraday rotator 14 for a net rotationof 90°. The Faraday rotator 14 will rotate the light one half of theangular difference between the two polarizers 12 so that when lighttraveling in the opposite direction of the arrows of laser beam 18 meetsthe originally first polarizer 12, at an angle of incidence that isperpendicular to the polarizer's pass axis thereby completely blockingback reflections through the optical isolator 10. In effect, an opticalisolator 10 serves as an optical diode or check valve.

FIG. 1B shows a polarization selective circulator assembly comprising afour port optical device 11 providing polarization selective directingof a laser beam. A laser beam 18 enters a polarizing beam splitter 17 inwhich polarized light aligned to one axis of the polarizing beamsplitter 17 passes through, and polarized light not aligned to that oneaxis of the polarizing beam splitter 17 is rejected. For the portion oflaser beam 18 which passes through polarizing beam splitter 17, it thenpasses through a Faraday rotator 14 followed by a half waveplate 16 fordirecting the laser beam 18. Another polarizing beam splitter 17 may beused to direct the remaining portion of laser beam 18 again into twosub-components comprising gain path laser beam 13 and aperture pathlaser beam 15. Four port optical device 11 may be used to providesteering of components of laser beam 18 in various directions within anoptical circuit while minimizing back reflections in the oppositedirection of the arrows of laser beam 18.

FIG. 2 shows a first configuration of a laser light source 20 capable ofpreventing optical feedback and varying the intensity variation of thelaser light by rotating a half waveplate. Laser light source 22 maycomprise a single longitudinal light source operated at continuous waveoperation. The emitted laser beam 18 from the laser light source 22inherently possesses a significant polarization and a half waveplate 16permits the aligning of that polarization along a predetermined axis ofpolarizer 12 contained within optical isolator 10. Such a configurationis used to minimize any parasitic, undesirable back reflection of laserlight into the laser light source 22 which may result in deleteriouseffects such as wavelength drift and linewidth broadening of the laserlight source 22. The half waveplate 16 may be angularly aligned tovarious angles of incidence of the polarizer 12 to vary the intensity ofthe laser beam 18 which exits laser light source 22 and travels throughthe optical system. Those skilled in the art will recognize that anumber of optical isolators 10 may be used to decrease even further thepossibility of back reflected light into the laser light source 22 bycascading several optical isolators 10.

FIG. 3 shows another configuration of a laser light source 30 capable ofpreventing optical feedback and varying the intensity variation of thelaser light using an electro-optic modulator. An optical isolator 10 mayalso be used to prevent reflected light from entering into the laserlight source 22 and causing undesired effects as described above.Additionally, an intensity modulator 32 may be placed either in front ofthe optical isolator 10 or after for modulating the intensity of thelaser beam 18 as it then travels through the remainder of an opticalcircuit. Those skilled in the art will recognize a wide variety ofelectro-optic modulators which will serve the function of intensitymodulator 32 including Pockels effect elements utilizing the linearelectro-optic effect and Kerr effect elements utilizing the quadraticelectro-optic effect. A very common material candidate for a Pockelscells is Lithium niobate (LiNbO₃).

FIG. 4 shows one possible embodiment of the invention comprising a fourpass, dual [could be a single or “n” rods; two is just an example] rodlaser media pulsed laser light source 40. Note, however, that source 40may be formed of one or many rods or slabs, as other considerations maydictate. This embodiment shows laser light source 20 which emits laserbeam 18 which passes through an isolator 10 and is directed using afirst mirror 42 through a beam expander 46 for broadening the beam waistof laser beam 18 to minimize the divergence of laser beam 18 as itpropagates through free space given its inherently Gaussian nature.Laser beam 18 then passes through four port optical device 11 in whichlaser beam 18 is directed to the gain path laser beam 13. The gain pathlaser beam 13 passes through two high-gain laser media 48 and thenthrough a phase correction plate and a Faraday rotator 49 where thelinear polarization of laser beam 18 is rotated 45°.

Typically, high-gain laser media 48 comprising glass materials such asNd³⁺:glass are inherently amorphous and non-birefringent whereassemiconductor materials such as Nd³⁺:YAG might degrade the polarizationstate of laser beam 18 as it passes through them. In the case ofoptically birefringent high-gain laser media, the use of an adjustablewaveplate for phase compensation can improve the performance of thesystem. The phase correction plate compensates for any rodbirefringence, which may be thermally induced. The high-gain laser media48 may comprise any material that will sustain an inverted population ofenergy states when optically pumped. It then reflects off of an endmirror 44 which then passes through the Faraday rotator 49 again whereit is rotated an additional 45°, orthogonal to the polarization of theoriginal laser beam 18.

The reflected laser beam then passes a second time through high-gainlaser media 48 and into four port optical device 11 where it is directedinto the direction of aperture path laser beam 15 and then reflects offa mirror 42 where it passes through an aperture 43 which helps tominimize self-oscillations caused by amplified spontaneous emissionsfrom the gain medium. Beam 15 reflects at end mirror 44 retracing itspath back into four port optical device 11 where it is again directed topass through high-gain laser media 48 for a third pass. It then travelsthrough to Faraday rotator 49 and to end mirror 44 where it is againreflected and retraces its path in passing through Faraday rotator 49,being converted into the same linear polarization as the original gainpath laser beam 13 in its first pass through high-gain laser media 48.The gain path laser beam 13 then passes a fourth time through high-gainlaser media 48 and into the four port optical device 11 where it isdirected to pass out in the direction of output laser beam 19.

Each high-gain optical medium 48 is optically pumped to generate aninverted population of energy states using a pumping array 51 of laserdiodes driven by a diode driver 52 which delivers electric current tooperate the laser diodes of the pumping array 51. To generate pulses ofamplified laser light, a trigger signal 54 is used to drive the diodedriver 52 which operates the pumping array 51 of laser diodes. The pulserate at which the output laser beam 19 may be pulsed is governed mostdirectly by the frequency of the trigger signal 54 which is used topulse the diode driver 52. The switching of the optical pumping resultsin gain switching of the high-gain laser medium 48 which serves toprovide for a pulsing of the conditions in which laser lightamplification may occur. The result is an amplified, pulsed output laserbeam 19.

FIG. 5 shows another possible embodiment of the invention comprising,for example, a dual pass, dual rod laser media pulsed laser light source50. This embodiment is strikingly similar to four pass, dual rod lasermedia pulsed laser light source 40. The main difference is that thereexists no four port optical device 11 is replaced by a single polarizingbeam splitter 17.

FIG. 6 shows another possible embodiment of the invention comprising,for example, a four pass, single slab laser medium pulsed laser lightsource 60. This embodiment shows laser light source 20 which emits laserbeam 18 which is isolated from feedback with isolator 10 and is directedthrough beam expander 46 for broadening the beam waist of laser beam 18to minimize the divergence of laser beam 18 as it propagates throughfree space given its inherently Gaussian nature. Laser beam 18 thenpasses through polarizing beam splitter 17 in which polarized lightaligned to one axis of the polarizing beam splitter 17 passes through,and polarized light not aligned to that one axis of the polarizing beamsplitter 17 is redirected in the direction of an output laser beam 19.For the portion of laser beam 18 which passes through polarizing beamsplitter 17, it then passes through a Faraday rotator 14 followed by ahalf waveplate 16 and a second polarizer 17 for directing the laser beam18 through high-gain laser media 48. Slab designs pass P-state with highefficiency because the face is almost at Brewsters Angle. The high-gainlaser media 48 may comprise any material that will sustain an invertedpopulation of energy states when optically pumped. It then reflects offof turning mirror 42. The reflected laser beam then reflects off of asecond turning mirror 42, passes a second time through high-gain lasermedia 48 and is directed into the direction of laser beam 15 and thenreflects off end mirror 44 before retracing its path back into thehigh-gain laser media 48 for a third pass. It then reflects off mirror44 to mirror 44 where it is again reflected and retraces its path inpassing through high-gain laser media 48 for a fourth time and into thepolarizer 17 and half waveplate for aligning the laser beam along apredetermined incidence angle. The laser beam 18 then passes a secondtime through Farady rotator 14. After traveling through Faraday rotator14, the laser beam 18 has been rotated to be orthogonal to the originallaser beam. Laser beam 18 enters polarizing beam splitter 17 where it isdirected to pass out in the direction of output laser beam 19.

In the present example, each high-gain optical medium 48 is opticallypumped to generate an inverted population of energy states using pumpingarrays 51 of laser diodes driven by a diode driver 52 which deliverselectric current to operate the laser diodes of the two pumping arrays51. To generate pulses of amplified laser light, a trigger signal 54 isused to drive the diode driver 52 which operates the pumping arrays 51of laser diodes. The pulse rate at which the output laser beam 19 may bepulsed is governed most directly by the frequency of the trigger signal54 which is used to pulse the diode driver 52. The switching of theoptical pumping results in gain switching of the high-gain laser medium48 which serves to provide for a pulsing of the conditions in whichlaser light amplification may occur. The result is an amplified, pulsedoutput laser beam 19.

FIG. 7 shows another possible embodiment of the invention comprising afour pass, dual slab laser medium pulsed laser light source 70. Thisembodiment is strikingly similar to four pass, single slab laser mediumpulsed laser light source 60, in that it uses the same number of diodesas in the FIG. 6 example. The main difference is that there exists asecond slab laser medium 48 and mirror assemblies 72 (either mirrors orcoatings) are placed on both of the high-gain laser media 48. This mayresult in a system that is more efficient (per diode) than the system ofFIG. 6, but with only a small added cost of the second slab.

FIG. 8 shows another possible embodiment of the invention comprising aneight pass, single slab laser medium pulsed laser light source 80. Thisembodiment is strikingly similar to four pass, single slab laser mediumpulsed laser light source 60. The main difference is that there exists aplurality of turning mirrors 42 to direct the laser beam through thehigh-gain laser media 48 eight times and mirror assembly 72 (eithermirrors or coatings) is placed on the high-gain laser media 48.

FIG. 9A shows another possible embodiment of the present invention. FIG.9A includes remote seeding of amplifier 54 with a fiber optic link.Laser light source 20 emits laser beam 18 which is isolated fromfeedback with optical isolator 10 and directed into input couplingoptics and polarization preserving single-mode fiber optics.Polarization preserving single-mode fiber optic is coupled to amplifier54, which includes output coupling optics. Amplifier 54 (not includingoutput coupling optics) is equivalent to the amplifier section shown inFIG. 6. Amplifier methods illustrated in FIG. 4, 5, 7 or 8 can be used,as well. Long-term stability of amplifier 54 is improved by de-couplingof laser beam 18. If laser beam 18 laser “walks” but part of the lightstill couples into the polarization preserving single-mode fiber optic,then output of amplifier 54 may only drop a small amount.

FIG. 9B shows another possible embodiment of the present invention. FIG.9B represents remote seeding of amplifier with internal modulators.Laser light source 20 emits laser beam 18 which is isolated fromfeedback with optical isolator 10. Laser beam 18 is input in amplitudemodulator, phase modulator and other beam/laser conditioning components.Prior to being input to polarization preserving single-mode fiberoptics, laser beam 18 is input to input coupling optics. Laser beam 18is output from polarization preserving single-mode fiber optics tooutput coupling optics. Laser beam 18 is amplified in a manner similarto that shown in FIG. 6. Amplifying methods illustrated in FIG. 4, 5, 7or 8 can be used, as well.

The present invention provides several benefits including minimizingthermal heating of the high-gain laser medium by using laser diodes toperform the optical pumping. Using laser diodes which operate within avery narrow wavelength regime minimize optically-induced thermal heatingof the laser medium in that little electromagnetic radiation outside ofthe desired spectrum bombards the high-gain laser medium as withconventional methods.

Using laser diodes for optically pumping the high-gain laser mediumprovides additional benefits including permitting a faster pulse rate,variable pulse rate. By performing optical amplification and pulsing inthe manner described above, the present invention also minimizesamplification induced distortion of a seed laser beam. Consequently, thephysical properties of the original seed laser beam are maintained inthe resultant amplified, output beam. Many applications includingoptical interferometry require a highly amplified, spectrally pureoutput laser beam which the present invention will provide. To broadenthe dynamic detection range of an optical interferometer employing thepresent invention, the intensity of the output laser beam may also bemodulated.

FIG. 10 shows an exemplary embodiment of the amplification system asused in an optical interferometry application. In this exemplaryapplication, an ultrasound testing system 90 has a sonic energygenerator 92, a laser source 94, and a detection device 96. However, thesystem may be envisaged in various configurations with some, all, ornone of these items. For example, the system 90 may have a laser source94 and a detection device 96.

The sonic energy generator 92 may take various forms. These forms mayinclude a laser or a transducer, among others. The sonic energygenerator may generator a sonic energy wave in the object 98. Forexample, a laser may direct a coherent beam of electromagnetic energy atthe object 98. The electromagnetic energy may impart heat energy to theobject causing a thermal expansion. As a result of the expansion, asonic energy wave may by produced in the object 98. However, other meansmay be employed to produce a sonic energy wave such as transducers, orapplied stress, among others.

The laser source 94 may take the forms as described above. Referring toFIGS. 1B, and 4-8, the beam 19 may be directed at an object. Returningto FIG. 10, the beam may be directed at an object 98. The beam may bemodulated by the sonic energy wave propagating about the object 98.Further, the beam may reflect from the object to become a modulatedreflected beam 102.

The modulated reflected beam 102 may be collected in a detection device96. The detection device may, for example, be an interferometer. Theinterferometer may take many forms. These forms may include aFabry-Perot interferometer, a two-wave mixing interferometer, and a dualdifferential confocal Fabry-Perot interferometer, among others. However,various alternate detection devices may be envisaged including agas-coupled laser acoustic detector and others.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade hereto without departing from the spirit and scope of the inventionas described by the appended claims.

1. A system for measuring sonic energy in a test object, the systemcomprising: a sonic energy generator, the sonic energy generatoroperable to generate the sonic energy about the test object; a source ofcoherent electromagnetic energy, the source of coherent electromagneticenergy directing at least one pulse of electromagnetic energy at thetest object, the source of coherent electromagnetic energy furthercomprising: a low amplification seed laser light source; and anamplification component, the low amplification seed laser light sourcetransmitting at least one low amplification seed laser light signal tothe amplification component, the amplification component furthercomprising: at least one amplification medium with time varying opticalgain driven by a time varying signal, the low amplification seed laserlight signal traversing the at least one amplification medium; and atleast one pumping diode, the at least one amplification mediumamplifying the low amplification laser light signal by stimulatingemissions of a population inversion provided by the at least one pumpingdiode to generate an amplified laser light signal, wherein the amplifiedlaser light signal varies with the time varying optical gain; and aninterferometer, the interferometer collecting at least one scatteredlight signal produced by a scattering from the object of the at leastone pulse of electromagnetic energy with sonic energy in the testobject, wherein the interferometer outputs a signal representative ofthe sonic energy in the test object; and a processor operable to processthe output of the interferometer to obtain information about an internalstructure of the test object.
 2. The system of claim 1 wherein the lowamplification seed laser light signal is amplified by repeatedlytraversing the at least one amplification medium.
 3. The system of claim1, the system further comprising: a generation laser beam applied to thetest object coaxially with the at least one pulse of electromagneticenergy.
 4. The system of claim 1, wherein the source of coherentelectromagnetic energy further comprises: an isolator associated withthe low amplification seed laser light source, the isolator isolatingthe low-amplification seed laser light source from the amplificationcomponent.
 5. The system of claim 1 wherein the at least oneamplification medium is a diode pump rod.
 6. The system of claim 1wherein the at least on e amplification medium is a diode pump slab. 7.A method for measuring a sonic energy signal about a test object, themethod comprising: generating sonic energy about the test object;generating a low amplification seed laser light signal from alow-amplification seed laser light source; transmitting saidlow-amplification seed laser light signal to an amplification component,wherein the amplification component has a time varying optical gaindriven by a time varying signal; amplifying said low-amplification seedlaser light signal in said amplification component by stimulatingemissions of the population inversion provided by a pumping diode togenerate an amplified laser light signal, wherein the amplified laserlight signal varies with the time varying optical gain; directing saidamplified laser light signal to test object; collecting a scatteredlight signal associated with said amplified laser light signal in aninterferometer; and processing an output of the interferometer to obtaininformation about an internal structure of the test object.
 8. Themethod of claim 7, wherein said amplifying step further comprisesrepeatedly directing said low-amplification seed laser light signalthrough said amplification medium.
 9. The method of claim 7, whereinsaid directing step further comprises the step of extracting saidlow-amplification seed laser light signal from an isolator associatedwith said amplifier.
 10. The method of claim 7, further comprising thestep of isolating said amplification component from the source of saidlow-amplification seed laser light signal.
 11. The method of claim 7,wherein said amplification component comprises a diode pump rod.
 12. Themethod of claim 7, wherein said amplification component comprises adiode pump slab.
 13. The method of claim 7, wherein said generating stepfurther comprises the step of generating a continuous wavelow-amplification seed laser light signal, and further wherein saidamplifying step further comprises the step of amplifying said continuouswave low-amplification seed laser light signal to generate saidamplified laser light signal having a maximum gain of a predeterminedgain coefficient times the amplification of said continuous wavelow-amplification seed laser light signal.
 14. The method of claim 7,further comprising the step of isolating said low-amplification seedlaser light source from said amplification component for preventingfeedback into said low-amplification seed laser light source.
 15. Themethod of claim 7, further comprising the step of isolating saidamplification component from said output destination for extracting saidamplified laser light signal from said amplification source.
 16. Themethod of claim 7, further comprising the steps of: transmitting saidlow-amplification seed laser light via an optical fiber to an isolatorthat interfaces with said amplification component; and transmitting saidamplified laser light signal through an optical fiber from saidamplification component to said output destination.
 17. The method ofclaim 7, wherein said generating step further comprises the step ofgenerating microsecond pulses of said low-amplification seed laser lightsignal.
 18. The method of claim 7, wherein said generating step furthercomprises the step of generating variable pulse rate pulses of saidlow-amplification seed laser light signal.
 19. A system for measuring asonic energy signal associated with a test object, the systemcomprising: a sonic energy generator, the sonic energy generatoroperable to generate the sonic energy about the test object; a seedlaser light source for providing a leaser beam with a desired linewidth;at least one optical isolation assembly placed in the path ofpropagation of the laser beam for preventing reflected laser lightfeedback into the seed laser light source; a polarization selectiveassembly aligned in the path of propagation of the laser beam fordirecting a first polarization state of the laser beam in a firstpropagation direction and at least one additional polarization state ofthe laser beam in at least one additional propagation direction; atleast one pumping array comprising a multiplicity of laser diodesdistributed across at least one high-gain laser medium having a timevarying optical gain driven by a time varying signal, wherein the atleast one high-gain laser medium aligned in the path of propagation ofeither the first polarization state or the at least one additionalpolarization state of the laser beam for optically pumping the at leastone high-gain laser medium for generationg a population inversion ofenergy states within the at least one high-gain laser medium foramplifying the laser beam and generating an output pulse of laser lightthat varies with the time varying optical gain; and an interferometer,the interferometer collecting scattered laser light associated with theoutput pulse of laser light, wherein the sonic energy in the test objectscatters the output pulse of laser light and wherein the interferometerouptuts a signal representative of the sonic energy in the test object;and a processor operable to process the output of the interferometer toobtain information about an internal structure of the test object. 20.The system of claim 19, further comprising: a diode driver fordelivering an electric current to the multiplicity of laser diodes foroptically pumping the at least one high-gain laser medium; and a triggersignal for pulsing the optical pumping of the high-gain laser medium bydelivering the trigger signal to the diode driver for switching theelectric current delivered to the at least one pumping array.